During the course of gravitational bending effected by means of a ring mould, it has been visually observed that, after the glass begins to bend and the central area of glass begins to "sag" downwards, this movement has been accelerating. Naturally, this acceleration is partially a result of the continuous rise of glass temperature but, nevertheless, this does not completely explain this high acceleration of movement. All in all, the bending of glass occurs very quietly until the "sagging" of the central area commences. Thus, the cooling must be commenced quickly and exactly at a correct moment. In the most preferred case, the bending should be stopped by cooling even before the glass is carried into another section or station. After a sufficient cooling, the glass can be carried forward without a risk of excessive bending.
Particularly, when using a hinged bending mould, it is possible that a lot of heat must be applied to certain areas and, thus, the temperature of these areas may rise above the tempering temperature.
An object of the present invention is to provide a method for bending and tempering a glass sheet, said method being capable of preventing the excessive bending of a glass sheet, eliminating the temperature differences in glass, and carrying a bent glass sheet to a tempering process at a sufficiently high temperature.
In order to achieve this object, a method of the invention is characterized in that, after a bending operation, the glass temperature is allowed to decrease to approximately 500.degree.-550.degree. C. throughout followed by increasing the overall glass temperature to a tempering temperature by heating as rapidly and uniformly as possible.
A method of the invention is capable of eliminating the temperature difference resulting in glass from bending in a simple manner without necessarily knowing the temperature profile in advance. A requirement for this is, however, that cooling and re-heating occur throughout at a constant rate (the coefficient of heat transfer must be as constant as possible). In a re-heating operation, the temperature increase must also be as rapid as possible in view of the process. The faster the re-heating is, the less additional bending occurs in glass during a heating operation .